Producing fuels from renewable sources has become increasingly important as a means of reducing the production of greenhouse gases and of reducing the imports of petroleum. See L. D. Gomez, C. G. Steele-King, S. J. McQueen-Mason, New Phytologist, 178, 473-485, (2008). Lignocellulosic biomass is typically made up of cellulose, hemicellulose, and lignin. These biomass components are non-edible, carbohydrate-rich polymers that may serve as a renewable source of energy. They typically make up to approximately 75% of the dry weight of biomass. As such, conversion of these non-edible biomass components into bio-fuels is of ongoing interest that can benefit the environment and reduce petroleum imports. See A. Demirbas, Energy Sources, Part B: Economics, Planning and Policy, 3(2) 177-185 (2008).
Currently, several approaches are available for converting biomass into fuels. For example, chemical processing routes may involve high temperature pyrolysis or biomass liquefaction; pyrolysis products (syngas) can be converted via Fischer-Tropsch chemistry to higher carbon number fuels. Biological routes typically first hydrolyze the polysaccharide content of biomass to monosaccharides using cellulase enzymes. These monosaccharides are then microbially converted to fuels.
Early efforts to biologically produce fuels from biomass included the fermentation of both starch-derived and lignocellulosic-derived carbohydrates to bio-alcohols such as bio-ethanol and bio-butanol. See Blanch, H. W. and C. R. Wilke, Sugars and Chemicals from Cellulose, Reviews in Chemical Engineering, eds., N. E. Amundson and D. Luss, vol 1, 1 (1982). These natural biological routes to produce alcohols (e.g., ethanol and butanol) from carbohydrates typically yield low molecular weight compounds that are generally more suitable as gasoline additives than as jet and diesel fuel compounds. While advances in metabolic engineering have enabled biological production of several higher molecular weight jet and diesel fuel compounds, these processes typically suffer from low titers and yields.
More recent efforts have focused on the carbohydrate source obtained from lignocellulosic biomass. Cellulose and hemicellulose obtained from lignocellulosic biomass after pre-treatment and hydrolysis affords hexoses and pentoses, respectively. Subsequent dehydration of these sugars into furfural and 5-hydroxymethylfurfural (HMF) may be achieved by chemical processes. Biological routes can ferment the hexoses and pentoses to short chain alcohols (e.g., ethanol and butanols) or to higher carbon number alkanes and alkenes, terpenes and fatty acids that can be esterified for use as diesel fuels.
While there are efficiencies of hexose and pentose conversion to short-chain alcohols, current microbial routes to higher carbon number products often have low yields and product titers. These products are currently not economically attractive as fungible fuels that can be employed as gasoline, jet and diesel fuel additives or replacements.
Thus, what is needed in the art is a commercially-viable process of producing fungible fuels, such as transportation fuels, and other chemicals from biomass, which allows for the control of product selectively. Moreover, what is needed in the art is a commercially-viable process for producing higher molecular weight fuel compounds and other chemicals from a saccharide-derived from biomass, such as an acetone-butanol-ethanol (ABE) mixture.